Covalent Bonds
What are covalent bonds?
Covalent Bonds
• A covalent bond is formed when
neutral atoms share one or more
pairs of electrons.
Covalent Bonds
• Covalent bonds form between two or
more non-metal atoms (Groups 14-17).
• Covalent bonded atoms form individual
molecules.
• Covalent compounds can be weak
solids, liquids, or gases.
Molecular Compounds
• A molecule is a neutral group of atoms
that are held together by covalent bonds.
• A chemical compound whose simplest
units are molecules is called a molecular
compound.
• Elements in a molecular compound can
form multiple substances depending on
the numbers of each element.
CH4 methane
C2H2 acetylene
How are bond length &
bond energy related?
Formation of a Covalent Bond
• The distance between two bonded atoms
at their minimum potential energy is the
bond length.
• Bond energy is the energy required to
break a chemical bond and form neutral
isolated atoms.
Bonds that have the higher bond
energies
(stronger bonds) have the shorter bond
lengths.
How does the octet rule
apply to covalent bonds?
Formation of a Covalent Bond
Octet Rule
• Bond formation follows the octet rule:
Covalent compounds share electrons so
that each atom has an octet of
electrons (8) in its highest
energy level.
Exceptions to the Octet Rule
• Hydrogen forms bonds in which it is
surrounded by only two electrons.
• Boron has just three valence electrons,
so it tends to form bonds in which it is
surrounded by six electrons.
What are the 3 types of
covalent bonds?
Single Covalent Bonds
• When atoms share one pair of electrons,
each atom contributes one electron to the
bond.
• Called a single covalent bond, each atom
gains the use of one electron for each single
bond.
Double Covalent Bonds
In a double covalent bond atoms share two
pairs of electrons, each atom contributes two
and gains two electrons in the bond.
In a triple covalent bond atoms share three
pairs of electrons, each atom contributes three
and gains three electrons in the bond.
How are molecules
modeled?
Lewis Structures
• Electron-dot notation can also be used to
represent the structure of molecules, called
Lewis Structures.
• The pair of dots between the two symbols
represents the shared electron pair of the
hydrogen-hydrogen covalent bond.
H
H
Lewis Structures
• The pair of dots between the two symbols
represents the shared pair of a covalent bond.
• In addition, each fluorine atom is surrounded
by three pairs of electrons that are not shared
in bonds.
• An unshared pair, also called a lone pair, is a
pair of electrons that is not involved in bonding
and that belongs exclusively to one atom.
What are the steps for
drawing Lewis Structures?
Drawing Lewis Structures
Step 1
• Get a covalent formula and draw the
electron-dot notation for each atom in the
formula.
H2O
H H
O
Drawing Lewis Structures
Step 2
• From the electron-dot notation determine
how many bonds each atom needs to
get an octet.
H2O
Drawing Lewis Structures
Step 3
• Determine the central atom (Skip this
step if there are only 2 atoms):
• Carbon is always the central atom
• If there is more than 1 carbon they
make a chain in the center.
-C-C-C• Hydrogen & group 17 elements are
never in the center.
• The least electronegative is in the
center.
Drawing Lewis Structures
Step 4
• Spread the remaining elements around
the central atom & bond each to the
center with a single bond.
H2O
H-O-H
Drawing Lewis Structures
Step 5
• Compare the number of bonds that each
element has with the number of bonds
each element needed. Add double or
triple bonds as needed.
H2 O
H-O-H
Drawing Lewis Structures
Step 6
• Add unshared electrons to any elements
from groups 15, 16 or 17.
• Valence electrons - bonds made =
unshared electrons
H2 O
H-O-H
How do covalent
compounds share
electrons?
Polar & Nonpolar Covalent Bonds
• Bonds in which atoms share electrons equally
are called nonpolar covalent bonds.
• Electronegativity difference between the two
atoms is below 0.3
Polar & Nonpolar Covalent Bonds
A polar covalent bond is a covalent bond
between atoms in which the electrons are
shared unequally.
The more electronegative atom
attracts electrons more strongly
and gains a slightly negative
charge. The less electronegative
atom has a slightly positive
charge.
Polar & Nonpolar Covalent Bonds
Electronegativity difference between the two
elements is below 2.0 to 0.3.
• The electron is
held closer to
the more
electronegative
atom in the
molecule.
Identifying Bond Type
Which type of bond (nonpolar
covalent, polar covalent or ionic) will
form between each of the following
pairs of atoms?
a. N and H
b. F and F
c. Ca and Cl
d. Al and Cl
How is polarity indicated on
Lewis Structures?
Polar & Nonpolar Covalent Bonds
The symbol  is used to mean partial
charge.
+ is used to show a partial
positive charge
– is used to show a partial
negative charge
Polar & Nonpolar Covalent Bonds
The O—H bonds in a water molecule are
polar.
• The highly electronegative oxygen
partially pulls the electrons away from
hydrogen.
How are covalent
compounds named?
Naming Diatomic Molecular Compounds
Diatomic molecular compounds: contain two
covalently bonded atoms of the same
element
The Diatomic 7: H2 N2 O2 F2 Cl2 Br2 I2
The molecule name is the same as the name
of the element. The diatomic 7 do not exist
as single atoms in nature. They are always in
the diatomic form unless they are in a
compound with other elements.
Naming Binary Molecular Compounds
Binary molecular compounds: contain two
different covalently bonded elements
Rule 1. The center element’s name is written
first. If there is no central atom write the name
of the less electronegative element first.
Rule 2. The second element in the name: add
the suffix -ide to the name of the element.
Rule 3. Add prefixes to indicate the number of
atoms of each element in the chemical
formula.
Exception: the prefix "mono-" is only used on
the first word for oxygen.
Prefixes for Naming Covalent Compounds
What are the properties of
covalent compounds?
Properties of Molecular Compounds
• They melt and evaporate at low
temperatures, many are liquid or gas at
room temperature.
• Solids are weak and can be broken or
ground into a powder easily.
• Not electrical conductors in the solid or
liquid state.
• Often have an odor because of low
boiling point
Melting and Boiling Points of Compounds
Properties of Substances with Metallic, Ionic, and
Covalent Bonds
Intermolecular Forces
What are intermolecular forces?
Intermolecular forces are weak forces of
attraction between some covalent
molecules.
These attractions are responsible for
determining whether a molecular
compound is a gas, a liquid or a solid at
room temperature.
Intermolecular Forces
How do the strengths of
intermolecular attractions compare
to intramolecular forces: ionic and
covalent bonds?
Intermolecular attractions are weaker
than either ionic or covalent bonds.
Intermolecular Forces
The two weakest attractions between
molecules are collectively called van der
Waals forces.
• Van der Waals forces consist of
dipole interactions and dispersion
forces.
Intermolecular Forces
Dispersion forces, the weakest of all
intermolecular forces, are caused by the
motion of electrons.
• They occur even between nonpolar
molecules.
• Moving electrons momentarily move to
one side of a molecule, their electric force
influences the neighboring molecule’s
electrons to momentarily move to the
opposite side.
Intermolecular Forces
Dipole interactions occur when
polar molecules are attracted to one
another.
*The electrical attraction occurs
between the oppositely charged
regions of polar molecules.
*Dipole interactions are similar to, but
much weaker than, ionic bonds.
Intermolecular Forces
Hydrogen bonds are strong attractive
forces in which a hydrogen covalently
bonded to a very electronegative atom is
also weakly bonded to an unshared
electron pair of another electronegative
atom.
Intermolecular Forces
Hydrogen bond